The standard for WLAN (wireless local area network) technology is being developed as IEEE (institute of electrical and electronics engineers) 802.11 standard. IEEE 802.11a/b uses an unlicensed band on 2.4 or 5 GHz. The IEEE 802.11b provides a data rate of 11 Mbps, while the IEEE 802.11a provides a data rate of 54 Mbps. IEEE 802.11g applies OFDM (orthogonal frequency division multiplexing) on 2.4 GHz to provide a data rate of 54 Mbps. IEEE 802.11n applies MIMO-OFDM (multiple input multiple output-OFDM) to provide a data rate of 300 Mbps for 4 spatial streams. The IEEE 802.11n supports channel bandwidths up to 40 MHz. In this case, a data rate of 600 Mbps is provided.
Meanwhile, a communication system for allowing an operation of an unlicensed user on a frequency band defined to be used by a licensed user is currently discussed. In this case, a frequency temporarily unused by the licensed user can be called a whitespace, and more particularly, a whitespace on a TV band can be called a TV whitespace (TVWS).
Currently, IEEE 802.11af standard is being developed to regulate an operation of an unlicensed device on TVWS (TV whitespace) band.
TVWS includes VHF (very high frequency) bands (e.g., 54˜60 MHz, 76˜88 MHz, 174˜216 MHz) and UHF (ultra high frequency) band (e.g., 470˜698 MHz) assigned for TV broadcasting, which mean the frequency bands allowed to be used for unlicensed devices under the condition that communications of licensed device (e.g., TV broadcasts, wireless microphones, etc.) operating on the corresponding frequency bands are not interrupted.
On 512˜608 MHz or 614˜698 MHz, operations are allowed for all unlicensed devices except several special cases. Yet, the bands 54˜60 MHz, 76˜88 MHz, 174˜216 MHz and 470˜512 MHz are allowed for the communications between fixed devices only. In this case, the fixed device means the device configured to perform a transmission at a determined location only. In the following description, a whitespace band includes the aforementioned TVWS, by which the present invention may be non-limited.
An unlicensed device desiring to use the whitespace band should provide a protection function for a licensed device. Hence, before a transmission is started on the whitespace band, it is mandatory to check whether a licensed device occupies the corresponding band. In particular, a use of an unlicensed device can be allowed only if a licensed device is not in use on a whitespace band.
To this end, an unlicensed device accesses a GDB (geo-location database) (or TV band database) through Internet or dedicated network and should obtain information on a channel list available for the corresponding area. In this case, the GDB is the database for storing and managing information on licensed devices registered therein and channel use information that dynamically changes depending on geo-locations and use times of the corresponding licensed devices. In order to solve the problem of co-existence between unlicensed devices that use the whitespace, such a signaling protocol as a common beacon frame and the like, a spectrum sensing mechanism and the like are available.
In IEEE 802.11 system, TVWS user equipment may indicate an unlicensed device that operates using IEEE 802.11 MAC (medium access control) layer and PHY (physical) layer. In the present specification, a station (STA) indicates a TVWS user equipment, which operates on TVWS spectrum, unless described separately.
The STA should provide a function of protecting an incumbent user or a primary user. In this case, the incumbent or primary user includes a licensed user (e.g., a TV user, a wireless microphone, etc.) and means a user to which a priority access is granted. In particular, in case that the incumbent user is using TVWS, the STA should stop using a corresponding channel. Hence, the STA discovers a channel (i.e., a channel not used by a licensed device) available for an unlicensed device and then should operate on the available channel.
A method for an STA to discover an available channel may include one of a scheme of performing a spectrum sensing mechanism, a scheme of discovering a TV channel schedule by accessing a GDB, and the like. For the spectrum sensing mechanism, it is able to utilize one of an energy detection scheme (i.e., if a strength of a received signal is equal to or greater than a predetermined value, determining that an incumbent user is using a corresponding channel), a feature detection scheme (i.e., if a digital TV preamble is detected, determining that an incumbent user is using a corresponding channel), and the like. Subsequently, the STA should check whether a corresponding channel is used by a licensed device at a corresponding location in a manner of accessing a GDB and then acquiring GDB information based on its location information. In doing so, the access to the GDB and the information acquisition should be performed frequently enough to protect the licensed device.
If it is determined that the incumbent user is using a channel right adjacent to a currently used channel through the spectrum sensing scheme or the GDB, a user equipment (or STA) and a base station (or an access point (AP) can protect the incumbent user in a manner of lowering a transmission power.